Seals are essential for the proper operation of a wide variety of devices including, but not limited to, turbines and compressors. Accordingly, seals find uses in a wide variety of locations. Specifically, their use at the interface between a turbine or compressor shaft and one or more stationary parts adjacent to the shaft are important to both the efficiency and operation of such devices. As with any mechanical device, continued use and wear results in a number of issues and/or cumulative negative effects. For example, leakages create a substantial effect on engine specific fuel consumption (SFC). For example, an average SFC increase of over 1% annually in large turbofan engines can generally be attributed to the wear and erosion of the seals. Leakage due to internal-flow systems of these engines has accounted for up to a 17% loss in power and over 7% increases in SFC.
The classical sealing technology for gas turbines or compressors used rigid seals such as cylindrical, labyrinth, or honeycomb seals. Presently, the labyrinth seal is the most common type of shaft seal with a combination of honeycomb seals and labyrinth seals used primarily as blade tip seals. The effectiveness of these types of seals depends on the radial clearance between the rotating and stationary parts as well as the number of seal stages, these factors being the main design conundrum for rigid seals. While a small clearance assures better sealing, differential thermal expansion, or dynamic excursions of the shaft associated with operational maneuvers, start-up or coast-down cause rubbing and wear damage at the interface between the stationary and rotating areas. Consequently, interstage or other type of leakage may increase with a resulting decrease in the engine efficiency. With linear speeds of near 1000 ft/sec, such rubbing contact and resulting interface wear will produce steady deterioration that may result in possible catastrophic failure.
Mitigation of such an environment occurs only if the seal interface becomes compliant and is able to follow the blade tip or shaft surface in its excursion/movement without being damaged thus avoiding the above mentioned consequences to the seal leakage effectiveness.
One of the first successful compliant seals with applications for the high temperature, high speed, and high pressure environment of a gas turbine/compressor involved the brush seal introduced in the early to mid-1980s. While a brush seal provides a good answer to compliance requirements and has become a serious competitor to labyrinth seals, there are also negative aspects engendered by the nature of the mechanics and operation of same. One such aspect involves the mounting of the brush seal with a significant preload onto a shaft, thus creating significant interface frictional and wear issues, as well as the significant heating of the bristle tips, sometimes to such temperatures that the tips get welded to the shaft. Another significant problem involves the dislodging of a bristle from the brush pack and its eventual entrainment within the power stream. Thus, while a brush seal does provide the compliance needed, it also brings about full contact with a rotating surface thereby resulting in a number of detrimental operational and structural problems.
Accordingly, there is a need in the art for improvement upon the brush seal whereby one maintains the advantages of the brush seal while improving efficiency.